Composite materials are made from two or more constituent materials with significantly different physical or chemical properties that, when combined, produce a material with characteristics different from the individual components. Composite materials are used across many industries, including aircraft manufacturing. Adhesive bonding of composite parts in aircraft structures can be more desirable than other joining methods due to higher joint stiffness and superior fatigue performance for the bonded segment.
In order to obtain high quality, strong adhesive bonds, there are two main considerations: proper surface preparation and the choice of adhesive/adherend. A freshly cleaned surface is required, as the surface may absorb gases, release agents, or other contamination that inhibit bonding. Further, the adhesive must spread out or “wet” the surface of the adherend (i.e., the composite substrate). Wetting is mostly controlled by the surface energy of the adhesive and adherend. In order for an adhesive to wet the adherend, the surface energy of the substrate must be higher than the surface energy of the adhesive. Roughness can increase the surface energy, creating a better surface for bonding. Roughness requirements can be satisfied by abrasive surface preparation techniques and proper cleaning prior to bonding. However, in order to avoid time- and labor-intensive abrasive surface preparation methods, such as sanding and grit blasting, peel ply surface preparation has become a common method of producing acceptable bonding surfaces for some applications.
A peel ply is a woven synthetic fabric that is added as a layer to an adherend substrate and cured to the substrate prior to adhesive bonding. In some peel ply methods, during curing, the viscosity of a resin (e.g., epoxy resin, polyester resin, vinyl ester resin, bismaleimides resin, etc.) in the preimpregnated (“prepreg”) substrate drops and allows the resin to flow and impregnate the peel ply between the fibers and in gaps where warp and weft meet. This imprints the channeled characteristics of the peel-ply material on the composite surface. After curing, the peel ply is then removed from the substrate surface immediately before bonding or analysis. In other peel ply methods (e.g., vacuum assisted resin transfer molding), unimpregnated (“dry”) fibers are infused with resin after they have been placed on the substrate. While traditional peel ply surface preparation methods leave behind traces of the peel ply fiber (e.g., polymers) on the substrate surface that inhibit adhesive bonding at room temperature, these methods have proven to be successful in achieving acceptable surface chemistries when bonding composite parts at elevated temperatures.